The man accused of leading the gang rape of a student on a New Delhi bus has been found hanged in his prison cell.

Police say Ram Singh took his own life in the high-security Tihar jail where he had been on suicide watch in an isolated cell.

But the 33-year-old suspect's family say he was killed and an investigation into the death is under way.

Singh's father, Mangelal Singh, said his son had been raped in prison by other inmates and had been repeatedly threatened by inmates and guards.

He said his son gave no hint of any despair that could drive him to take his own life when he visited him in jail four days ago.

Image Caption:Days of protests followed the attack

And he insisted his son also had a badly injured hand and would have been unable to hang himself.

"Somebody has killed him," he said.

Mangelal Singh said he feared for the safety of another son who is also on trial over the rape.

Vimla Mehra, the director general of the jail, refused to comment on the circumstances surrounding the death.

"The inquiry is being conducted and it would be premature to make any statement about the details of the incident," he said.

The death has angered the victim's family, who said the authorities had been negligent and they had been denied justice.

Her father, who cannot be named for legal reasons, said: "We don't understand how could the police fail to protect Ram Singh?

Image Caption:Rallies grew violent and water cannon were turned on protesters

"They knew he was the prime accused in my daughter's case.

"How could they let him choose the way he wanted to die? The police have failed and I wonder what will happen to the case now."

Singh was accused of leading a gang of five others who subjected the 23-year-old physiotherapy student to a two-and-a-half hour sex attack in which she was gang raped.

Her internal injuries were so horrific she died two weeks later in a hospital in Singapore despite surgery to try to save her.

Singh's death comes in the middle of his trial. He and four others are accused of luring the woman and a male friend, who were on their way home from a trip to the cinema, on to the bus where they beat him before repeatedly raping her.

A senior police officer on the case said: "The case will continue. There is no reason for the case to suffer."

Lawyers for the men have previously accused police of beating confessions out of their clients.

Singh was the regular driver of the white private bus, usually used to carry school children, which is alleged to have been used in the December 16 attack.

Police say he had been out with friends and drinking heavily when they decided to take the bus out for a night-time joyride.

The men are facing charges of murder, rape, and kidnapping, and if found guilty are expected to be given the death penalty.

A sixth suspect aged 17 is being tried in a juveniles' court, where he faces a maximum three years imprisonment.

The attack caused worldwide revulsion and outrage in India. Days of protests followed in a country where a woman is raped every 20 minutes.

The woman's friend, who is still suffering from the injuries he sustained in the attack, has already given evidence at the trial. His account was said to be a crucial part of the case.